In the formation of integrated circuits (IC's), thin films containing metal elements are often deposited upon the surface of a substrate, such as a semiconductor wafer. Thin films are deposited to provide conducting and ohmic contacts in the circuits and between the various devices of an IC. For example, a desired thin film might be applied to the exposed surface of a contact or via on a semiconductor wafer, with the film passing through the insulative layers on the wafer to provide plugs of conductive material for the purpose of making interconnections across the insulating layers.
Particularly, certain aspects of IC components are degraded by exposure to the high temperatures normally related to traditional thermal chemical vapor deposition (CVD) processes. At the device level of an IC, there are shallow diffusions of semiconductor dopants which form the junctions of the electrical devices within the IC. The dopants are often initially diffused using heat during a diffusion step, and therefore, the dopants will continue to diffuse when the IC is subjected to a high temperature during CVD. Such further diffusion is undesirable because it causes the junction of the device to shift, and thus alters the resulting electrical characteristics of the IC. Therefore, for certain IC devices, exposing the substrate to processing temperatures of above 800.degree. C. must be avoided, and the upper temperature limit may be as low as 650.degree. C. for other more temperature sensitive devices.
Furthermore, such temperature limitations may become even more severe if thermal CVD is performed after metal interconnection or wiring has been applied to the IC. For example, many IC's utilize aluminum as an interconnection metal. However, various undesirable voids and extrusions occur in aluminum when it is subjected to high processing temperatures. Therefore, once interconnecting aluminum has been deposited onto an IC, the maximum temperature to which it can be exposed is approximately 500.degree. C., and the preferred upper temperature limit is 400.degree. C. Therefore, as may be appreciated, it is desirable during CVD processes to maintain low deposition temperatures whenever possible.
In one particular application, a thin film of titanium must be deposited over silicon contacts prior to depositing the metal interconnection in order to provide low contact resistance. In state-of-the-art integrated circuits, this titanium deposition is followed by the deposition of a titanium nitride (TIN) diffusion barrier layer. The diffusion barrier prevents aluminum interconnection from diffusing into the silicon contact and causing a short circuit. The diffusion barrier is also needed in the case of tungsten metalization. In this case, the TiN diffusion barrier prevents the tungsten precursor, tungsten hexafluoride, from diffusing into the silicon contact and reacting with the silicon.
There are low temperature physical techniques available for depositing titanium nitride (TIN) on temperature sensitive substrates. Sputtering is one such technique involving the use of a target of layer material and an ionized plasma. To sputter deposit a film, the target is electrically biased and ions from the plasma are attracted to the target to bombard the target and dislodge target material particles. The particles then deposit themselves cumulatively as a film upon the substrate. Titanium Nitride may be sputtered, for example, over a silicon substrate after various contacts or via openings are cut into a level of the substrate.
The conformality of sputtered TiN is poor in the high aspect ratio contacts which are used in state-of-the-art integrated circuits. This poor conformality leads to poor diffusion barrier properties. Therefore, TiN films deposited by chemical vapor deposition (CVD) are preferred over sputtered films. TiN films deposited by CVD from TiCl.sub.4 and NH.sub.3 provide 100% conformality over very high aspect ratio contacts.
The HCl which is a byproduct of this TiN deposition reaction can etch the titanium contact layer. In order to avoid etching of the titanium, the upper layer of the titanium must be nitrided prior to TiN CVD. This nitridization can be done by rapid thermal annealing at 800.degree. C. in a nitrogen or ammonia ambient. This temperature is far too high for the shallow junctions which are present in most state-of-the-art integrated circuits. Therefore, a low temperature nitridation process is necessary. Annealing the titanium film in ammonia at 650.degree. C. results in a very poor quality nitridation of the titanium surface as evidenced by the lack of a characteristic gold color. When TiN is deposited by chemical vapor deposition over the thermally annealed titanium, the result is the same as with the unannealed titanium. Prior attempts at annealing using an ammonia plasma have been unsuccessful.